NAME¶
pam_afs_session - AFS PAG and token PAM module
SYNOPSIS¶
auth optional pam_afs_session.so
session required pam_afs_session.so
DESCRIPTION¶
The AFS session service module for PAM, typically installed at
/lib/security/pam_afs_session.so, establishes new AFS sessions and
obtains AFS tokens when a new session is opened for a user. It is a shared
object that is dynamically loaded by the PAM subsystem as necessary, based on
the system PAM configuration. PAM is a system for plugging in external
authentication and session management modules so that each application doesn't
have to know the best way to check user authentication or create a user
session on that system. For details on how to configure PAM on your system,
see the PAM man page, often
pam(7).
This module provides pam_setcred, pam_open_session, and pam_close_session
implementations for AFS. Because pam_setcred is part of the auth PAM group, it
also implements a dummy pam_authenticate that always succeeds (otherwise, it
can't provide a pam_setcred).
Make sure that this module is NEVER listed as "sufficient" or as the
only "required" module in the auth group. Doing so will potentially
allow users to log on without any password. There unfortunately isn't a way to
work around this and still provide pam_setcred without running afoul of a bug
in (at least) Linux PAM 0.99.7.1 and probably earlier that causes
authentication to fail when the final module in the auth group returns
PAM_IGNORE and "[default=done]" was given as the action.
Here are the actions of this module:
- pam_open_session
- When a new session is opened, this module will first check to see if AFS
is running on the system. If not, it will log a message and exit
successfully. If AFS is running, it will place the user's session in a new
PAG (Process Authentication Group, often implemented as supplemental
groups, which limits user tokens to only processes in that PAG). It will
then attempt to obtain tokens, either directly if built with the Heimdal
libkafs library and Kerberos support or by running an external
aklog program. If PAG creation fails, the module will fail; if
obtaining tokens fails, the module will log a warning but still return
success.
The module will only attempt to obtain tokens if the environment variable
KRB5CCNAME is set in the environment, unless otherwise configured (see the
always_aklog option). It will always create a new PAG, however.
- pam_close_session
- If and only if pam_open_session successfully obtained AFS tokens and AFS
is still running on the system, pam_close_session will delete the tokens
in the current PAG (equivalent to running unlog). To leave the
tokens after session close, set the retain_after_close option.
- pam_setcred
- When pam_setcred is called with the PAM_ESTABLISH_CRED flag, it will do
the same as if pam_open_session was called. When pam_setcred is called
with the PAM_DELETE_CRED flag, it will do the same as if pam_close_session
was called. When called with the PAM_REINITIALIZE_CRED flag or the
PAM_REFRESH_CRED flag, it won't create a new PAG but instead will only
attempt to get new tokens (still skipping this unless KRB5CCNAME is set in
the environment or always_aklog is set).
This module is primarily intended for use with a Kerberos v5 authentication
module. It does not itself do any user authentication; it cannot, for
instance, be used to authenticate users to a
kaserver. While it is
intended for use with an
aklog that uses Kerberos v5 ticket caches to
obtain tokens, it can be used with any
aklog implementation
(always_aklog may have to be set if no Kerberos v5 ticket cache will be
present).
This module performs no authorization checks and does not hook into password
changes; it only implements the session functions and pam_setcred.
CONFIGURATION¶
The AFS session PAM module supports the following configuration options, which
may be set in the PAM configuration as arguments listed after
"pam_afs_session.so" or in the system
krb5.conf.
Some of them take arguments, in which case the argument will be given after
"=". The rest are boolean options. To set a boolean option in the
PAM configuration, just give the name of the option in the arguments. To set
an option that takes an argument, follow the option name with an equal sign
("=") and the value, with no separating whitespace. Whitespace in
option arguments is not supported in the PAM configuration files of most PAM
implementations.
To set an option for the PAM module in the system
krb5.conf file, put
that option in the [appdefaults] section. The AFS session PAM module will look
for options either at the top level of the [appdefaults] section or in a
subsection named "pam-afs-session" (currently, realm-specific
configuration is not checked). For example, the following fragment of a
krb5.conf file would set
aklog_homedir to true and
minimum_uid to 100.
[appdefaults]
aklog_homedir = true
pam-afs-session = {
minimum_uid = 100
}
There is no difference to the PAM module whether options are specified at the
top level or in a "pam-afs-session" section; the
"pam-afs-session" section is supported in case there are options
that should be set for the PAM module but not for other applications. For more
information on the syntax of
krb5.conf, see
krb5.conf(5).
If the same option is set in
krb5.conf and in the PAM configuration, the
latter takes precedent. Note, however, that due to the configuration syntax,
there's no way to turn off a boolean option in the PAM configuration that was
turned on in
krb5.conf.
- afs_cells=cell[,cell...]
- Obtain tokens for the listed cells instead of the default local cell. If
more than one cell is listed, try to obtain tokens for each cell. If
specified in krb5.conf, the cells can be separated by any
combination of spaces and commas; if specified in the PAM configuration,
they must be separated by commas.
If the AFS session PAM module is running an external program, this option is
implemented by passing a -c flag with the cell as its argument for
each listed cell to that program. If aklog_homedir is also set, the
-c flags and the -p flag will all be passed to the external
program.
- aklog_homedir
- Try to obtain the necessary tokens to access the user's home directory. If
the libkafs token-obtaining API is used, setting this will cause the AFS
session PAM module to pass the user's home directory into that API and
request that the appropriate tokens be obtained. If running an external
aklog program, aklog will be called with -p
home-directory where home-directory is the home directory of
the local user for which the session is being opened or refreshed. This
generally will tell aklog to check that path, find all AFS cells
involved in access to that path, and attempt to obtain tokens for each
one. Note that this means that if the user's home directory is not in AFS,
no tokens will be obtained.
In either case, the user's home directory is obtained via getpwnam()
based on the username PAM says we are authenticating.
- always_aklog
- Normally, the AFS session PAM module only tries to obtain tokens if
KRB5CCNAME is set in the PAM environment. If this option is set, it will
always attempt to obtain tokens. This is only useful if it is configured
to run an external aklog program.
This can be used if your environment doesn't correctly set KRB5CCNAME in the
environment for some reason, or if your aklog doesn't rely on a
Kerberos ticket cache to obtain tokens (or can find the cache on its own
via some other means).
- debug
- If this option is set, additional trace information will be logged to
syslog with priority LOG_DEBUG.
- ignore_root
- If this option is set, the AFS session PAM module won't take any action
(and will exit successfully) if the account for which the session is being
established is named "root".
- kdestroy
- If this option is set and the AFS session PAM module was built with
Kerberos support, the user's ticket cache will be destroyed after tokens
are obtained successfully. If tokens are not obtained successfully, the
ticket cache will be left intact. Please note that this is not properly a
security feature, since the ticket cache will still be written to disk
between the time the Kerberos PAM module authenticates the user and the
time the AFS session PAM module is run. It can, however, be used to reduce
the window during which Kerberos ticket caches are lying about if the only
use one has for ticket caches is to obtain AFS tokens.
- minimum_uid=uid
- If this option is set, the AFS session PAM module won't take any action
(and will exit successfully) if the account for which the session is being
established has a UID lower than uid.
- nopag
- If this option is set, no PAG will be created. Be careful when using this
option, since it means that the user will inherit a PAG from the process
managing the login. If sshd, for instance, is started in a PAG,
every user who logs in via ssh will be put in the same PAG and will share
tokens if this option is used.
This is the default on Mac OS X, where PAGs are not supported.
- notokens
- If this option is set, the AFS session PAM module will only create a PAG
and not attempt to obtain tokens. Setting this option overrides all other
settings related to acquiring tokens, including always_aklog. If both
nopag and notokens are set, the module essentially does nothing.
Setting notokens also implies retain_after_close, meaning that the AFS
session PAM module will also not attempt to delete tokens when the user's
session ends.
- program=path
- The path to the aklog program to run. Setting this option tells the
AFS session PAM module to always run an external program to obtain tokens
and never use the libkafs interface, even if the latter is available.
You may pass options to this program by separating them with commas (or
spaces or tabs in krb5.conf or if the configuration syntax of your
PAM configuration allows this). For example, the setting:
program=/usr/bin/aklog,-noprdb,-524
will run "/usr/bin/aklog -noprdb -524" as the program to obtain
tokens. The arguments are passed directly, not parsed by the shell.
If this option is not set, the default behavior is to call the libkafs
function to obtain tokens, if available, and otherwise to use a default
path to aklog determined at compile time (the first aklog
found on the compiler's path by default). If no aklog could be
found at compile time and libkafs isn't used, this option must be
set.
- retain_after_close
- If this option is set, pam_close_session will do nothing (successfully)
rather than deleting tokens. This will allow programs started in the
user's PAG that are still running when the log out to continue to use the
user's tokens until they expire. Normally, the AFS kernel module will
automatically clean up tokens once every process in that PAG has
terminated.
ENVIRONMENT¶
- KRB5CCNAME
- This module looks for KRB5CCNAME in the PAM environment and by default
does not run aklog if it is not set.
The entire PAM environment is passed to
aklog as its environment (rather
than the environment of the process running the PAM functions).
WARNINGS¶
As mentioned above, this module implements a dummy pam_authenticate function so
that it can provide a pam_setcred function. Never list this module as
"sufficient" or as the only "required" module or you may
allow users to log on without a password.
While spawning an external
aklog program, the AFS session PAM module
resets the SIGCHLD signal handler to the default handler while the program
runs and then restores it afterward. This is done to avoid having aklog
interfere with process handling in the calling application, but it means that
there's a race condition that can cause children to be incorrectly handled if
they exit while aklog is running. There is unfortunately no good solution to
this other than building against Heimdal and using the libkafs interface to
obtain tokens instead of an external program.
To detect whether AFS is running on the system, the AFS session PAM module
temporarily sets a SIGSYS handler before attempting an AFS system call. That
handler may also modify a static variable. Neither of these should ideally be
done in a PAM module, but there is no other good way of checking for the
non-existence of a system call that doesn't crash the application on some
operating systems. The PAM module will attempt to restore the previous SIGSYS
handler, if any, after the test is done, and the static variable is used in
such a way that running it from multiple threads shouldn't be an issue, but be
aware that the PAM module is doing this behind the back of the application and
may interfere with unusual SIGSYS handlers or similar application actions.
NOTES¶
When using the libkafs interface to obtain tokens, be sure that it is configured
properly for the type of AFS tokens expected at your site. As of Heimdal 0.7,
the default behavior is to contact the krb524 service to translate Kerberos v5
tickets into Kerberos v4 tickets to use as tokens. AFS cells running current
server software no longer need this, and if your site doesn't run the krb524
service, this may break token acquisition.
Sites running AFS servers that understand Kerberos-v5-derived tokens should add
configuration like:
libkafs = {
EXAMPLE.ORG = {
afs-use-524 = no
}
}
to the [appdefaults] section of their
krb5.conf files to disable use of
the krb524 service. See the Heimdal
kafs(3) man page for more
information.
SEE ALSO¶
aklog(1),
kafs(3),
pam(7),
syslog(3),
unlog(1)